Histidine-rich glycoprotein (HRG) is an alpha 2-glycoprotein that was first described by Heimberger, et al, in 1972. Today, HRG is generally regarded as a mild prothrombotic protein. Blood samples of 585 individuals were collected with the aid of the Alameda-Contra Costa Medical Association (ACCMA) Blood Bank, Oakland, CA. Sex, age, ethnic origin, and blood-type information were available for each sample. The blood was processed to isolate the cell free plasma, and plasma HRG concentration was measured relative to that of a normal pool through a modified Laurell technique. Among Caucasian individuals, the mean HRG level of blood-type AB subjects, 125+/−28%, was found to be significantly greater than the means for subjects with A and O blood-types, 103+/−35% and 105+/−30% respectively ( P=.0246). In addition, the average HRG level appears to increase linearly with age. The mean plasma level of HRG in subjects 50–59 years old was significantly greater than the level in subjects 30–39 years old ( P=.0020). The correlation observed between blood-type and plasma HRG level in this study supports previously reported results that indicate significant genetic control over the plasma level of this protein. The age and blood-type based correlations observed in this study raise the question of whether these variables need be addressed if HRG level were to be employed in a clinical setting as a diagnostic tool. Copyright © 1996 Elsevier Science Ltd